Laser tracker

ABSTRACT

Some embodiments of the invention include a coordinate measuring machine for detecting the position and alignment of a spatially movable measuring aid. The coordinate measuring meaching may include a retroreflector: a base; a support, which is fixed on the base rotatably about a first rotation axis; a beam directing unit, which is fixed to the support rotatably about a second rotation axis, substantially orthogonal to the first rotation axis; means for detecting a rotation angle of the support relative to the base; and means for detecting a rotation angle of the beam directing unit relative to the support. In some embodiments, in the beam directing unit comprises a laser emission and reception optical unit and a first optical distance measuring unit having at least one first distance measuring device for measuring the distance to a retroreflector of the measuring aid by means of a first measurement radiation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/962,877, filed Dec. 8, 2015, which claims priority to Europeanapplication no. 14197717.3, filed Dec. 12, 2014. The foregoingapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a laser tracker having at least two distancemeasuring devices integrated into the targeting unit, wherein a firstdistance measuring device is designed for continuously tracking a targetpoint, and a second distance measuring device is designed for a distancemeasurement by means of a pulsed or frequency-modulated light or laserbeam, in particular designed for a distance measurement by means ofwaveform digitizing (WFD) or as a frequency-modulated, in particularcoherent, continuous wave laser (FMCW laser radar).

BACKGROUND

Measuring apparatuses designed for continuously tracking a target pointand coordinatively determining the position of said point can generallybe combined under the term laser tracker particularly in associationwith industrial measurement. In this case, a target point can berepresented by a retroreflective unit (e.g. cube prism) which istargeted by an optical measurement beam of the measuring apparatus, inparticular a laser beam. The laser beam is reflected back to themeasuring apparatus in a parallel fashion, the reflected beam beingdetected by a detection unit of the apparatus. In this case, an emissiondirection and respectively a reception direction of the beam areascertained, for example by means of sensors for angle measurement whichare assigned to a deflection mirror or a targeting unit of the system.In addition, with the detection of the beam, a distance from themeasuring apparatus to the target point is ascertained, e.g. by means oftime-of-flight or phase difference measurement or by means of the Fizeauprinciple.

Laser trackers according to the prior art can additionally be embodiedwith an optical image detection unit with a two-dimensional,light-sensitive array, e.g. a CCD or CID camera or a camera based on aCMOS array, or with a pixel array sensor and with an image processingunit. In this case, the laser tracker and the camera can be mounted oneon top of another in particular in such a way that their positionscannot be altered relative to one another. The camera is arranged, forexample, in a manner rotatable together with the laser tracker about thesubstantially perpendicular axis thereof, but in a manner pivotable upand down independently of the laser tracker and thus, in particular,separately from the optical unit of the laser beam. Furthermore, thecamera—e.g. depending on the respective application—can be embodied aspivotable only about one axis. In alternative embodiments, the cameracan be installed in an integrated design together with the laser opticalunit in a common housing.

With the detection and evaluation of an image—by means of an imagedetection and image processing unit—of a so-called auxiliary measuringinstrument with markings whose relative position with respect to oneanother is known, it is thus possible to deduce an orientation of anobject (e.g. a probe) arranged on the auxiliary measuring instrument inspace. Together with the determined spatial position of the targetpoint, it is furthermore possible to precisely determine the positionand orientation of the object in space absolutely and/or relative to thelaser tracker.

The object whose position and orientation are measured by means of themeasuring instrument mentioned therefore need not be a measuring probeitself, for example, but rather can be the measuring aid. The latter, aspart of the measuring system, for the measurement, is brought into aposition that is mechanically defined relative to the target object orcan be determined during the measurement, wherein, by means of themeasured position and orientation thereof, it is possible to deduce theposition and, if appropriate, the orientation of the measuring probe,for example.

Such auxiliary measuring instruments can be embodied by so-calledcontact sensing tools that are positioned with their contact point on apoint of the target object. The contact sensing tool has markings, e.g.light points, and a reflector, which represents a target point on thecontact sensing tool and can be targeted by the laser beam of thetracker, the positions of the markings and of the reflector relative tothe contact point of the contact sensing tool being known precisely. Theauxiliary measuring instrument can also be, in a manner known to aperson skilled in the art, a, for example handheld, scanner equipped fordistance measurement for contactless surface measurements, the directionand position of the scanner measurement beam used for the distancemeasurement relative to the light points and reflectors arranged on thescanner being known precisely. A scanner of this type is described in EP0 553 266, for example.

For distance measurement, laser trackers from the prior art have atleast one distance measuring device, wherein the latter can be embodiede.g. as an interferometer. Since such distance measuring units canmeasure only relative changes in distance, in addition tointerferometers so-called absolute distance measuring devices areinstalled in present-day laser trackers. The interferometers used fordistance measurement in this context use primarily—on account of thelong coherence length and the measurement range made possiblethereby—HeNe gas lasers as light sources. In this case, the coherencelength of the HeNe laser can be a few hundred meters, such that theranges required in industrial metrology can be obtained with relativelysimple interferometer constructions. A combination of an absolutedistance measuring device and an interferometer for determining distancewith a HeNe laser is known from WO 2007/079600 A1, for example.

In addition, in modern tracker systems—increasingly in a standardizedmanner—an offset of the received measurement beam from a zero positionis ascertained on a fine targeting sensor. By means of this measurableoffset, it is possible to determine a difference in position between thecenter of a retroreflector and the impingement point of the laser beamon the reflector and it is possible to correct or readjust the alignmentof the laser beam depending on this deviation in such a way that theoffset on the fine targeting sensor is reduced, in particular is “zero”,and the beam is thus aligned in the direction of the reflector center.As a result of the readjustment of the laser beam alignment, continuoustarget tracking of the target point can be carried out and the distanceand position of the target point can be determined continuously relativeto the measuring instrument. The readjustment can be realized in thiscase by means of a change in alignment of the deflection mirror providedfor deflecting the laser beam, said deflection mirror being movable in amotorized manner, and/or by pivoting of the targeting unit having thebeam-guiding laser optical unit.

For determining the orientation of the measuring aid, a detectiondirection of the camera is continuously aligned such that an image isdetectable in the direction of the tracking beam of the laser tracker.The camera can furthermore have a zoom function, wherein a magnificationlevel can be set depending on the determined distance between lasertracker and target point or measuring aid. With these two adaptationfunctions (alignment and magnification), the camera can thuscontinuously detect an image in which the measuring aid and inparticular the light points of the measuring aid are imaged. Anelectronically evaluatable, two-dimensional image of a spatialarrangement of light points arises as a result.

An image processing unit is provided for evaluating the image. This canbe used to identify the imaged light points, to determine the centroidsof the imaged light points and to determine the image coordinates ofsaid centroids, from which it is possible to calculate for example solidangles between the optical axis of the sensor, in particular thedetection direction, and the direction from the sensor to the respectivelight points.

Such a coordinate measuring machine having a laser tracker and an imagedetection unit for determining the position and orientation of objectsin space on which light points and reflectors are arranged is describedin U.S. Pat. No. 5,973,788, for example.

With the use of such coordinate measuring machines, at least three lightpoints that can be registered by the image detection unit and at leastone reflector that reflects the measurement beam of the laser trackerare arranged at the object whose position and orientation are to bedetermined, in positions that are known relative to the object. Thelight points to be registered by the image detection unit can be activelight sources (e.g. light-emitting diodes) or reflectors to beilluminated, wherein the light points are equipped or arranged in such away that they are unambiguously distinguishable from one another.

WO 2007/079600 A1 discloses a generic laser-based coordinate measuringmachine in which a light exit and light receiving optical unit of thedistance measuring apparatus, a measuring camera and an overview cameraare arranged on a common element, which is rotatable relative to atleast two axes, and a laser beam is coupled into the distance measuringapparatus by means of an optical waveguide from a laser module fittedoutside the beam directing unit.

By contrast, the measurement of distances without the use of measuringaids having a retroreflector, i.e. measurement directly to a surface ofan object to be measured, is not possible with such coordinate measuringmachines.

Accordingly, scanning an object surface is not possible either: in orderto detect objects or surfaces, use is often made of methods whichprogressively scan and in the process record the topography of astructure, such as e.g. of a construction site. In this case, such atopography constitutes a continuous sequence of points which describesthe surface of the object, or else a corresponding model or adescription of the surface. One conventional approach is scanning bymeans of a laser scanner which in each case detects the spatial positionof a surface point by the distance to the targeted surface point beingmeasured by the laser and this measurement being combined with the angleinformation of the laser emission. From this distance and angleinformation, the spatial position of the detected point can bedetermined and the surface can be continuously measured. In many cases,in parallel with this purely geometrical detection of the surface, imagerecording by means of a camera is also carried out, which, besides theoverall visual view, also provides further information, e.g. regardingthe surface texture. WO 97/40342 describes a method which records atopography by means of scanner systems installed in a stationary manner.In addition, scanning functions can be integrated into various otherinstruments as additional functions. WO 2004/036145 discloses, forexample, a geodetic measuring instrument which emits a laser beam fordistance measurement from its position within the detected range. Suchmeasuring instruments can likewise be modified for detecting surfaces ina scanning fashion, or be operated without modification. One examplethereof is motorized theodolites or total stations.

In order to provide such measuring and scanning functionalities that areusable without a retroreflector with generic coordinate measuringmachines such as laser trackers, solutions with attachment modules areknown from the prior art. By way of example, the document EP 2 620 745A1 discloses a measuring system consisting of a coordinate measuringmachine, e.g. laser tracker, and a scanning module to be fixed thereto.

Measuring distances without the aid of retroreflectors is for examplealso possible with the distance measuring instruments described in WO2006/040263 A1 or EP 1 869 397 B1, in which distances are ascertained bymeans of a frequency-modulated continuous wave radar (FMCW) or acoherent laser radar. However, these solutions lack a target trackingfunctionality.

However, this multi-component solution is, firstly, complex in terms ofproduction engineering and, secondly, unwieldy and impractical for theuser. It would therefore be advantageous to provide a coordinatemeasuring machine having both a target tracking functionality for aretroreflector and the possibility of ascertaining distances in a mannerfree of a measuring aid—i.e. in particular without a retroreflector.

SUMMARY

Therefore, some embodiments of the present invention provide an improvedcoordinate measuring machine that combines these functionalities in onemachine.

Furthermore, some embodiments to provide such a coordinate measuringmachine in which the structural construction is designed with lesscomplexity and better handleability, in particular without the necessityof a modular construction of the coordinate measuring machine.

Some embodiments of the invention extend a measuring instrumentaccording to the prior art in such a way that a scanning functionalityis additionally provided for the measuring instrument.

The invention firstly relates to a coordinate measuring machinecomprising a beam directing unit having a first and a second distancemeasuring device, wherein the measurement radiation of the firstdistance measuring device enables a target tracking of a retroreflector,while the measurement radiation of the second distance measuring deviceallows measuring of distances to surfaces that are reflective in ascattering fashion.

In this case, a first aspect of the invention relates to a laser trackerin which the second distance measuring device is designed as a WFDmodule for performing a waveform digitizing process.

Such a laser tracker according to the invention for positiondetermination and in particular continuous tracking of a target embodiedas a retroreflector or having at least one retroreflector comprises atleast a base, a support, which is fixed on the base rotatably about afirst rotation axis, a beam directing unit, which is fixed to thesupport rotatably about a second rotation axis, which is substantiallyorthogonal to the first rotation axis, means for detecting a rotationangle of the support relative to the base, and means for detecting arotation angle of the beam directing unit relative to the support. Inthis case, the beam directing unit comprises a laser emission andreception optical unit for emitting measurement radiation and forreceiving measurement radiation reflections, a first optical distancemeasuring unit having at least one first distance measuring device formeasuring the distance to a retroreflector of the measuring aid by meansof a first measurement radiation, and a second optical distancemeasuring unit comprising a second beam source for emitting a secondmeasurement radiation, a detector and a control and processing unit formeasuring a distance to a diffusely scattering surface of a targetobject by means of the second measurement radiation.

In accordance with the first aspect of the invention,

-   -   the second beam source is designed to emit a pulsed light        radiation, in particular a laser beam, as second measurement        radiation,    -   a portion of the second measurement radiation that is reflected        from the surface of the target object is forwardable to the        detector,    -   the detector is designed to detect at least one pulse of the        reflected portion of the second measurement radiation, and    -   the second optical distance measuring unit is designed to        digitize the detected pulse in the context of a waveform        digitizing process and to ascertain a distance to the surface of        the target object on the basis of the digitized pulse.

In one embodiment of the laser tracker in accordance with the firstaspect of the invention, a part of the second measurement radiation isforwardable to the detector as reference beam, in particular by means ofa first beam splitter provided in the beam path of the secondmeasurement radiation in the beam directing unit.

In a further embodiment of the laser tracker in accordance with thefirst aspect of the invention, said laser tracker comprises a beamreceiving unit, which is designed to guide reflected radiation thatentered the beam directing unit through the laser emission and receptionoptical unit to the second optical distance measuring unit, wherein thebeam receiving unit comprises a paraboloidal mirror having aparabolically designed mirror surface, and the paraboloidal mirror isdesigned to concentrate reflected radiation impinging on the mirrorsurface at a focal point, and in particular is designed to reflectreflected radiation impinging at a center of the mirror surface at anangle of 65° to 75°, in particular 70°. In this case, in particular

-   -   at the focal point a coupling-in element is provided for        coupling the concentrated reflected radiation into an optical        waveguide, in particular wherein a deflection element is        provided in the beam path between the paraboloidal mirror and        the coupling-in element, and/or    -   the paraboloidal mirror is arranged outside the common beam path        of the first measurement radiation and the second measurement        radiation, in particular wherein the beam receiving unit        comprises a beam splitting apparatus arranged in the common beam        path, by means of which beam splitting apparatus the reflected        radiation is directed onto the paraboloidal mirror.

A second aspect of the invention relates to a laser tracker in which thesecond distance measuring device is designed as an FMCW module forperforming a frequency evaluation of a frequency-modulated laser beamhaving a continuously varying frequency.

Such a laser tracker according to the invention for positiondetermination and in particular continuous tracking of a target embodiedas a retroreflector or having at least one retroreflector at leastcomprises a base, a support, which is fixed on the base rotatably abouta first rotation axis, a beam directing unit, which is fixed to thesupport rotatably about a second rotation axis, which is substantiallyorthogonal to the first rotation axis, means for detecting a rotationangle of the support relative to the base, and means for detecting arotation angle of the beam directing unit relative to the support. Inthis case the beam directing unit comprises a laser emission andreception optical unit for emitting measurement radiation and forreceiving measurement radiation reflections, a first optical distancemeasuring unit having at least one first distance measuring device formeasuring the distance to a retroreflector of the measuring aid by meansof a first measurement radiation, and a second optical distancemeasuring unit comprising a second beam source for emitting a secondmeasurement radiation, a detector and a control and processing unit formeasuring a distance to a diffusely scattering surface of a targetobject by means of the second measurement radiation.

In accordance with the second aspect of the invention,

-   -   the second beam source comprises drive means and is designed to        generate a frequency-modulated laser beam having a continuously        varying frequency as second measurement radiation,    -   a part of the second measurement radiation can be split off as        reference beam, in particular by means of a first beam splitter        provided in the beam path of the second measurement radiation in        the beam directing unit,    -   a portion of the second measurement radiation that is reflected        from the surface of the target object is forwardable to the        detector and    -   the second optical distance measuring unit is designed to        ascertain, on the basis of a frequency evaluation of the        reference beam and the measurement radiation reflection, a        distance to the surface of the target object.

In particular, ascertaining the distance is based on the principle ofthe evaluation of a phase difference between the frequency modulation ofthe reference signal and the frequency modulation of the reflectedmeasurement radiation signal.

In one embodiment of the laser tracker in accordance with the secondaspect of the invention, the second optical distance measuring unitcomprises a mixer, wherein the reference beam and the reflectedmeasurement radiation reflection are forwardable to the mixer, and themixer is designed for carrying out a homodyne or heterodyne mixingmethod. In this case, the second optical distance measuring unitcomprises, in particular, a low-pass filter disposed downstream of themixer.

In a further embodiment of the laser tracker in accordance with thesecond aspect of the invention, the second optical distance measuringunit comprises an optical reference system having a first optical fiber,a second optical fiber and a reference beam detector, wherein the firstand second optical fibers in each case have a known, mutually differentlength, and the reference beam is forwardable to the optical referencesystem. The two lengths differ from one another in particular in a ratioof at least 2:1.

A third aspect of the invention relates to a laser tracker in which thefirst or second distance measuring device is designed for distancemeasurement by means of a frequency comb.

Such a laser tracker according to the invention for positiondetermination and in particular continuous tracking of a target embodiedas a retroreflector or having at least one retroreflector, at leastcomprises a base, a support, which is fixed on the base rotatably abouta first rotation axis, a beam directing unit, which is fixed to thesupport rotatably about a second rotation axis, which is substantiallyorthogonal to the first rotation axis, means for detecting a rotationangle of the support relative to the base, and means for detecting arotation angle of the beam directing unit relative to the support. Inthis case, the beam directing unit comprises a laser emission andreception optical unit for emitting measurement radiation and forreceiving measurement radiation reflections, a first optical distancemeasuring unit having at least one first distance measuring device formeasuring the distance to a retroreflector of the measuring aid by meansof a first measurement radiation, and a second optical distancemeasuring unit comprising a second beam source for emitting a secondmeasurement radiation, a detector and a control and processing unit formeasuring a distance to a diffusely scattering surface of a targetobject by means of the second measurement radiation.

In accordance with the third aspect of the invention, the first opticaldistance measuring unit and/or the second optical distance measuringunit are/is designed for a frequency-comb-based orfrequency-comb-supported distance measurement.

One embodiment of the laser tracker according to the invention comprisesa scanning functionality, in the context of which the laser tracker isdesigned to carry out, by means of the second measurement radiation, amultiplicity of distance measurements with respect to a multiplicity ofmeasurement points on the surface of the measurement object, wherein thecontrol and processing unit is designed in such a way that for themultiplicity of the distance measurements, the rotation anglesrespectively detected are linked with the measured distance, such that apoint position is in each case defined by the linking, and a point cloudcomprising a number of the point positions is generatable.

A further embodiment of the laser tracker according to the inventioncomprises a calibration functionality for calibrating the first opticaldistance measuring unit and/or the second optical distance measuringunit, wherein the control and processing unit is designed in such a waythat the first measurement radiation and the second measurementradiation are emittable in a temporal relationship, in particularsimultaneously, onto a retroreflector, and the first optical distancemeasuring unit and/or the second optical distance measuring unit are/iscalibratable on the basis of a first distance to the retroreflector asmeasured by the first optical distance measuring unit and a seconddistance to the retroreflector as measured by the second opticaldistance measuring unit.

In one embodiment of the laser tracker according to the invention, thebeam directing unit comprises a position-sensitive detector, inparticular a tracking area sensor, for detecting the first measurementradiation or second measurement radiation reflected by a target, suchthat depending on a position of the reflected first or secondmeasurement radiation on the detector the alignment of the measurementradiation is readjustable for a continuous target tracking, inparticular wherein the beam directing unit additionally comprises alocalization camera for the coarse localization of the measuring aidand/or an overview camera for providing images for a user; the firstoptical distance measuring unit comprises an interferometer and/or anabsolute distance measuring device; the beam directing unit comprisesdeflection means, in particular beam splitters, for generating a commonbeam path of the first measurement radiation and the second measurementradiation in the beam directing unit, such that the first measurementradiation and the second measurement radiation are emittable through thesame laser emission and reception optical unit; and/or the laseremission and reception optical unit is designed as a laser emissionoptical unit and a laser reception optical unit separate from thelatter.

The invention additionally relates to a method for using a seconddistance measuring device designed as a WFD module in a laser trackerand a method for using a second distance measuring device designed as anFMCW module in a laser tracker.

A method for using a WFD module in a laser tracker for providingadditional measurement functionality, wherein the laser trackercomprises a beam directing unit having a laser emission and receptionoptical unit for emitting measurement radiation and for receivingmeasurement radiation reflections, and an optical distance measuringapparatus having at least one first distance measuring device formeasuring the distance to a measuring aid by means of a firstmeasurement radiation, comprises according to the invention:

-   -   generating a pulsed light beam, in particular laser beam, as        second measurement radiation by means of a beam source of the        WFD module,    -   emitting the second measurement radiation onto a target to be        measured, wherein one part of the second measurement radiation        is forwarded as reference beam to the detector, and another part        of the second measurement radiation is emitted through the laser        emission and reception optical unit,    -   a measurement radiation reflection of the second measurement        radiation that is reflected from the target and received through        the laser emission and reception optical unit is forwarded to        the detector,    -   the detector detects pulses of the forwarded reference beam and        of the reflected forwarded measurement radiation reflection,    -   the detected pulses are digitized in the context of a waveform        digitizing process of the WFD module, and    -   a distance to the target is ascertained on the basis of the        digitized pulses.

A method for using an FMCW module in a laser tracker for providingadditional measurement functionality, wherein the laser trackercomprises a beam directing unit having a laser emission and receptionoptical unit for emitting measurement radiation and for receivingmeasurement radiation reflections, and an optical distance measuringapparatus having at least one first distance measuring device formeasuring the distance to a measuring aid by means of a firstmeasurement radiation, comprises according to the invention:

-   -   generating a frequency-modulated laser beam having a        continuously variable frequency as second measurement radiation        by means of a beam source of the FMCW module,    -   emitting the second measurement radiation onto a target to be        measured, wherein one part of the second measurement radiation        is split off as reference beam, and another part of the second        measurement radiation is emitted through the laser emission and        reception optical unit    -   a portion of the second measurement radiation that is reflected        from the target and received through the laser emission and        reception optical unit is forwarded to a detector of the FMCW        module, and    -   on the basis of a frequency evaluation of the reference beam and        the reflected portion of the second measurement radiation, a        distance to the target is ascertained, in particular on the        basis of the principle of the evaluation of a phase difference        between the frequency modulation of the reference signal and the        frequency modulation of the reflected measurement radiation        signal.

In one embodiment of a method according to the invention, the target tobe measured is a diffusely scattering surface of a measurement object,wherein

-   -   the second measurement radiation is emitted onto the diffusely        scattering surface,    -   a portion of the second measurement radiation is reflected from        the surface, and    -   the distance to the surface is ascertained.

In one particular embodiment of this method, a multiplicity of distancemeasurements with respect to a multiplicity of measurement points on thesurface are carried out by means of the second measurement radiation,wherein for the multiplicity of the distance measurements a presentalignment of the beam directing unit is linked with the respectivelymeasured distance, such that a point position is in each case defined bythe linking, and a point cloud comprising a number of the pointpositions is generated.

In a further embodiment of a method according to the invention, thetarget to be measured is a retroreflector, in particular as part of ameasuring aid, wherein the second measurement radiation is emitted ontothe retroreflector, the second measurement radiation is reflected asmeasurement radiation reflection from the retroreflector, and thedistance to the retroreflector is ascertained.

In one particular embodiment of this method, the first measurementradiation and the second measurement radiation are emitted onto theretroreflector, wherein a distance measured by means of the firstmeasurement radiation and a distance measured by means of the secondmeasurement radiation are used for calibrating the first opticaldistance measuring device and/or the WFD module or the FMCW module, inparticular wherein the first measurement radiation and the secondmeasurement radiation are emitted simultaneously.

The invention furthermore relates to a computer program product forcarrying out method steps of a method according to the invention.

A computer program product having program code, stored on amachine-readable carrier, is designed according to the invention forcontrolling or carrying out at least the following steps of the methodaccording to the invention for using a WFD module, particularly if theprogram is executed on an electronic data processing unit designed as acontrol and processing unit of a laser tracker in accordance with thefirst aspect of the invention:

-   -   digitizing the detected pulses and    -   ascertaining the distance to the target.

A computer program product having program code, stored on amachine-readable carrier, is designed according to the invention forcontrolling or carrying out at least the following steps of the methodaccording to the invention for using an FMCW module, particularly if theprogram is executed on an electronic data processing unit designed as acontrol and processing unit of a laser tracker in accordance with thesecond aspect of the invention:

-   -   emitting the second measurement radiation, and    -   ascertaining the distance to the target on the basis of a        frequency evaluation of the reference beam and of the reflected        portion of the second measurement radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

The laser tracker according to the invention and the method according tothe invention are described in greater detail purely by way of examplebelow on the basis of specific exemplary embodiments that areillustrated schematically in the drawings, further advantages of theinvention also being discussed. In the figures, specifically:

FIG. 1 shows a laser tracker according to the invention when measuring adistance to a measuring aid;

FIG. 2 shows a laser tracker according to the invention when measuringthe distance to a measurement object by means of a WFD module;

FIG. 3 shows a laser tracker according to the invention in a frontalview;

FIG. 4 shows one embodiment of the laser tracker according to theinvention when scanning a measurement object in the context of ascanning functionality of the WFD module;

FIG. 5 shows an optical construction of a laser tracker from the priorart;

FIGS. 6a-b show a first embodiment of an optical construction of a lasertracker according to the invention comprising a WFD module;

FIGS. 7a-b show two embodiments of a beam receiving unit of the WFDmodule;

FIG. 8 shows components of one embodiment of a beam receiving unitcomprising a paraboloidal mirror in a three-dimensional view;

FIG. 9 shows a second embodiment of an optical construction of a lasertracker according to the invention comprising a WFD module;

FIG. 10 shows the embodiment from FIG. 9 when simultaneously measuring adistance to a retroreflector with the WFD module and further distancemeasuring devices;

FIG. 11 shows a first embodiment of an optical construction of a lasertracker according to the invention comprising an FMCW module;

FIG. 12 shows a second embodiment of an optical construction of a lasertracker according to the invention comprising an FMCW module; and

FIG. 13 shows an optical construction of a frequency-comb-based distancemeasuring unit.

DETAILED DESCRIPTION

FIG. 1 shows one exemplary embodiment of a coordinate measuring machineaccording to the invention, designed as a laser tracker 1. The lasertracker 1 shown comprises a base 40, a support 20 fitted thereon andhaving a handle 21, and a beam directing unit 10 mounted on two strutsof the support 20. The laser tracker 1 is arranged on a stand 45,comprises at least one first distance measuring device—in particular anabsolute distance measuring device (ADM) or an interferometer—(notillustrated here) and, by means of a laser beam 36, measures thedistance to a retroreflector 61 situated on a measuring aid 60. Themeasuring aid 60—embodied here by way of example as a measuringprobe—furthermore comprises a number of target markings 62, for examplein the form of reflective or self-luminous light points, and also ameasuring head 63 for positioning on a target point to be measured of atarget object 90.

The laser tracker 1 optionally comprises a measuring camera, which canbe designed in particular as a focusable camera system having variablemagnification in order to detect the target markings 62 arranged on themeasuring aid 60. The spatial alignment of the measuring aid 60 isdeterminable on the basis of the positions of the target markings 62that are recorded by the measuring camera.

A method—usable with such a measuring camera—for continuouslydetermining the spatial position of a measuring aid 60 having aplurality of target markings 62 in a fixed, known spatial distributionrelative to one another is described in EP 2 557 391 A1: said methodinvolves continuously detecting camera images of the target markings 62by means of a measuring camera having an area sensor comprising amultiplicity of pixels, and continuously carrying out reading passes inwhich the pixels are read with regard to a respective present exposurevalue. Furthermore, the method involves determining image positions ofthe imaged target markings 62 in the respective present camera image,and deriving the respective present spatial position of the measuringaid 60 on the basis thereof In this case, respective present regions ofinterest are continuously set on the area sensor depending on acollection of image positions determined in at least one previouslydetected camera image. The present image positions are then determinedexclusively taking account of only those present exposure values whichare obtained from pixels of the area sensor lying within the presentlyset regions of interest.

In order to identify and to be able to reproduce movements of themeasuring aid 60, such that the laser beam 36 remains aligned with theretroreflector 61, the laser tracker 1 comprises a position-sensitivedetector (PSD), in particular a tracking area sensor, such as isdescribed for example in WO 2007/079600 A1.

The PSD is preferably arranged in the beam directing unit 10 and makesit possible to readjust the alignment of the laser beam 30 by detectingthe alignment of the laser beam 30 reflected from a target, inparticular the retroreflector 61. The readjustment of the laser beamalignment makes it possible for continuous target tracking of the targetpoint to be carried out and for the distance and position of the targetpoint to be determined continuously relative to the measuring machine.

According to the invention, the laser tracker additionally has adistance measuring functionality that makes it possible to measure adistance to a surface 92 of the measurement object 90. This is shown inFIG. 2.

FIG. 2 shows the laser tracker from FIG. 1 when measuring a distance tothe measurement object 90 by means of a WFD measurement beam 76. Thedistance is measured by means of a waveform digitizing module (WFDmodule; not illustrated here) provided according to the invention in thebeam directing unit 10. This method allows distance measurements withoutusing a measuring aid 60 or a retroreflector.

FIG. 3 shows one exemplary embodiment of a laser tracker 1 according tothe invention in a frontal view. The laser tracker 1 comprises a base40, which is fixable on a holding apparatus, here illustrated in theform of a stand 45. A support 20 is fitted on the base 40 in a mannermounted rotatably about the vertical axis 9. The support 20 comprises afirst and a second strut, which project upward from a lower part of thesupport 20 and on which a beam directing unit 10 is mounted tiltablyabout the horizontal axis 8 by means of a shaft 25. A handle 21 for thetransport and handling of the laser tracker 1 by a user is fitted to thetwo struts at the top.

Both the mounting of the support 20 on the base 40 and the mounting ofthe beam directing unit 10 on the support 20 are preferably embodied asa fixed-movable bearing. Axial errors owing to temperature influencesand the resultant losses of accuracy are thus minimized. In addition, atemperature-governed expansion of the shaft 25 is noncritical and doesnot influence a strain of the bearing. As a result, the strain of thebearings remains constant over the entire temperature range of use.

The handle 21 can in particular be fixedly connected to the two struts,for example produced from a molding therewith or welded, adhesivelybonded or screwed thereto, such that it serves as an additionallystabilizing element for the struts, in particular with regard tobending. The handle 21 can advantageously be shaped in such a way thatit allows an exactly upwardly directed measurement, i.e. along thevertical axis 9, by means of the laser beam. Alternatively, the handle21 can also have an opening for passage of the laser beam at thecorresponding location.

A plurality of optical units are provided on the beam directing unit 10,including an optical unit 52 of a measuring camera and also a lens 50for the target tracking functionality with a laser emission andreception optical unit 51 of an optical distance measuring apparatus.Furthermore, the beam directing unit 10 preferably comprises an opticalunit of a localization camera for coarsely localizing the measuring aidand an optical unit of an overview camera for providing images for auser.

A laser module 30, preferably a helium-neon laser module (HeNe lasermodule), is integrated into the support 20, or into one of the struts.Particularly advantageous embodiments of a usable laser module 30 arealso disclosed in CH 706 633 A2.

An optical waveguide system comprising a first fiber 31 and a secondfiber 32 leads from said laser module 30 through the shaft 25 into thebeam directing unit 10 as far as a collimator 34 of a distance measuringapparatus (not illustrated here), in particular of an interferometer. Inthis case, the first fiber 31 of the optical waveguide system, saidfirst fiber running in the support 20, is connected without torsion tothe second fiber 32 of the optical waveguide system, said second fiberrunning in the beam directing unit 10, via a plug connection 33preferably provided in the support 20. Arranging the plug connection 33in proximity to the laser module 30 in the support 20 has the advantagethat the laser module 30 together with the first fiber 31 isexchangeable more easily. Preferably, the optical waveguide system ispolarization-maintaining, and/or the first and second fibers 31 and 32are single-mode fibers.

FIG. 4 shows one embodiment of the laser tracker 1 according to theinvention in which a scanning functionality is implementable by means ofthe WFD module. In this case, on a scanning area 94 of a surface 92 ofthe measurement object 90, a multiplicity of measurement points aresuccessively targeted by the beam directing unit 10, in each case adistance being ascertained and linked with angle data, such that aposition can be ascertained for each of the measurement points.Targeting the points can be carried out, as illustrated here, bytraversing the scanning area 94 in a regular pattern 96, e.g. inparallel paths at a defined distance from one another. The ascertainedpositions of the measurement points can then be joined together to forma point cloud.

FIG. 5 shows one exemplary optical construction 100 of a laser trackeraccording to the prior art, as described for example in EP 2 634 594 A1.It has a functionality for checking measurements of distance changesthat are carried out with an interferometer 12. Moreover, theconstruction 100 comprises a beam source 30, e.g. an HeNe laser beamsource or a laser diode, and an absolute distance measuring device 13(ADM) having a further beam source 131, e.g. a laser diode or an SLED(Superluminescent LED).

The light beam emerging from the beam source 131 of the ADM is guidedonto a polarizing beam splitter 133 and from there through anelectro-optical modulator 134 to a wavelength-dependent beam splitter150. Such a beam splitter with wavelength-dependent beam splitting isused in particular in the case of different emission wavelengths of thetwo light sources 30, 131. The returning light is guided in the ADM 13through the polarizing beam splitter 133 onto an ADM detector 132. Inparticular, in this context it is also possible to use other ADMarrangements and methods in which the measurement light beam can becoupled in and out through the wavelength-dependent beam splitter 150,for example. Examples of such a distance measuring device are describedin WO 03/062744 A1 and WO 2009/103172 A1. In principle, other types ofADM, such as e.g. phase measuring devices or distance measuring devicesoperating according to the Fizeau principle, can also be used here andin the other embodiments of the invention.

The interferometer 12 uses light that is generated by the beam source30. In the embodiment shown, said source 30 is assigned directly to theconstruction 100, wherein said source generates for example alongitudinally monomode laser radiation with a long coherence length(single frequency). In an alternative embodiment, the beam source 30 canbe assigned to a different component of the measuring instrument,wherein the radiation is coupled into the interferometer 12 by means ofan optical waveguide. The laser radiation generated is split into areference radiation 122 on a reference light path and a measurementradiation 36 on a measurement light path by a beam splitter 121. Themeasurement light path leads through an acousto-optical modulator 125and impinges together with the reference light path on a polarizing beamsplitter 126. The polarizing beam splitter 126 guides the measurementradiation 36 further to the wavelength-dependent beam splitter 150, anddirects the returning measurement light together with the referencelight via a polarization filter 123 to an interferometer detector 124.The method of operation of such an interferometer 12 is fundamentallyknown and is based on the wave interference principle. In particular, itis also possible to use other interferometer arrangements and methods inwhich the measurement radiation can be coupled in and out through thewavelength-dependent beam splitter 150, for example. One example of suchan interferometer is described in WO 03/062744 A1. In principle, othertypes of interferometers (e.g. Michelson with quadrature detection) canalso be used.

A superimposition of the reference radiation 122 with the measurementradiation 36 reflected at a movable target 61 and guided onto theinterferometer detector 124 is detected at the interferometer detector124. The intensity of the interference arising upon the superimpositionof the two radiations 36, 122 can be detected continuously (asinterferometer output variable) in this case. The derivation of theinterferometer output variable is based here at least on the detectedsuperimposition, wherein the interferometer output variable is dependenton a distance to the target.

If the target 61 is situated at a constant distance from the opticalconstruction 100 or from the interferometer detector 124, then theintensity value measured during the maintained fixed distance to thetarget 61 is constant. With a relative movement—in relation to anoptical axis defined by the measurement radiation 36—of the target 61with respect to the optical construction 100 (or with a movement of theconstruction), there is a change in the distance between the twocomponents 100, 61, thus a path difference between the referenceradiation 122 and the measurement radiation 36 and, as a result, theintensity measurable at the interferometer detector 124 depending on thedistance change. By means of the interferometer detector 124, theseintensity variations can be measured and detected (as an output variableprofile), in particular in a temporally resolved manner, and can be readand processed further for checking the correctness of such a distancechange measurement. The temporally resolved output variable profile isgenerated from the derived interferometer output variable, the distancechange being ascertained on the basis of the output variable profile.

In order to check the correctness of such a measurement, a movementparameter is continuously derived from the intensities detected by theinterferometer detector 124 and this parameter is continuously comparedwith a movement criterion. Depending on the comparison, informationregarding the reliability of the measurement carried out is then output.

The optical construction 100 furthermore comprises a λ/4 plate 140 and acomponent 160, which separate light that is incident in the construction100 from outside along a common optical axis used by the absolutedistance measuring device 13 and the interferometer 12, and coupled-outa first part of said light to an overview camera (not illustrated) and asecond part to a position transducer (not illustrated). The overviewcamera can have a dedicated optical unit and in addition an imageconverter. In this case, the overview camera typically has an apertureangle of around 10° and a focal length of 30-50 mm, for example, andserves for the coarse localization of measurement targets.

For detecting reflective targets, the construction 100 can additionallypreferably have a reflector illumination with a specific illuminationwavelength which illuminates an angular range that is preferably atleast equal to the aperture angle of the overview camera.

An evaluation electronic unit and/or evaluation software of the overviewcamera then detect(s) for example one or more bright light points in thefield of view of the overview camera, which in each case correspond to areflective target. It is possible to ascertain therefrom their positionin the image of the overview camera and therefrom in turn a change inthe alignment of the target, e.g. of an auxiliary measuring instrument(e.g. contact sensing unit or scanner), whereby the measuring instrumentor the optical construction 100 and the light beams of the distancemeasuring device or devices 12, 13 can be aligned with the target. Thus,an automatic target detection and a “lock-on”, i.e. a continuoustracking of the target, of the distance measuring devices 12, 13 onto atarget can therefore be realized.

The light portion for the position transducer is typically a beam ofreturning light that was emitted by one of the distance measuringdevices 12, 13, preferably by the interferometer arrangement 12. Theposition transducer can have a dedicated optical unit and, for example,a position-sensitive detector (tracking area sensor, in particular PSDor CCD), wherein measurement laser radiation reflected at the target canbe detected thereon.

In this context, PSD should be understood to mean an area sensor whichoperates locally in an analog manner and can be used to determine acentroid of a light distribution on the sensor area. In this case, theoutput signal of the sensor is generated by means of one or a pluralityof photosensitive areas and depends on the respective position of thelight centroid. By means of a downstream or integrated electronic unit,the output signal can be evaluated and the centroid can be ascertained.In this case, the position of the centroid of the impinging light pointcan be ascertained very rapidly (nanoseconds range) and with asub-nanometer resolution.

By means of said PSD, it is possible to determine an offset of theimpingement point of the detected beam from a servo control zero pointand to carry out a tracking of the laser beam to the target on the basisof the offset. For this purpose and in order to achieve a high accuracy,the field of view of said PSD is chosen to be comparatively small, i.e.corresponding to the beam diameter of the measurement laser beam.Detection using the PSD is carried out coaxially with respect to themeasurement axis, such that the detection direction of the PSDcorresponds to the measurement direction. The use of PSD-based trackingand fine targeting can take place only after the measuring laser hasbeen aligned with a retroreflective target (at least coarsely, i.e. insuch a way that the target lies within the measurement laser cone).

One exemplary optical construction of a laser tracker according to theinvention comprising an absolute distance measuring device 13 (ADM) anda waveform digitizing module (WFD module 70) according to the inventionis shown in FIGS. 6a and 6 b. In this case, FIG. 6a shows a distancemeasurement to a measurement object 90 by means of the WFD module 70according to the invention, and FIG. 6b shows a distance measurement toa retroreflector 61 by means of an ADM 13 such as was described in FIG.5.

The WFD module 70 is installed together with the ADM 13 in the beamdirecting unit of the laser tracker and uses the same laser emission andreception optical unit 51 as the ADM 13. The WFD module 70 comprises abeam source 71 for generating a pulsed light beam, in particular laserbeam, (WFD beam 76). The WFD beam 76 having measurement pulses isfirstly guided onto a first beam splitter 74, whereby a reference beam77 having the same measurement pulses as the WFD beam 76 is split offand is guided onto a reference beam coupling-in element 78, whichcouples the reference beam 77 into a first optical waveguide 79, whichleads to the detector 72 of the WFD module 70.

The other part of the WFD beam 76 is coupled into the common beam pathwith the measurement radiation of the ADM 13 by means of a second beamsplitter 75 and is directed through the laser emission and receptionoptical unit 51 onto the measurement object 90 to be measured. A portion81 of the WFD beam 76 that is reflected from the diffusely scatteringsurface of the measurement object 90 passes through the laser emissionand reception optical unit 51 again into the beam directing unit. Thereflected radiation 81 has target pulses which correspond to themeasurement pulses but are temporally offset—in accordance with thedistance to the target. In the beam directing unit, the reflectedradiation 81 is concentrated by a beam receiving unit 80 and directedonto a coupling-in unit 88, which couples the reflected radiation 81into a second optical waveguide 89, which leads to the detector 72 ofthe WFD module 70. The latter detects the reflected portion 81 of theWFD beam 76 and the reference beam 77. The pulses of the reflectedradiation 81 and of the reference beam 77 are digitized in a knownmanner referred to as waveform digitizing. The measurement pulses andtarget pulses digitized in this way are compared with one another, inparticular the temporal separation between them, and the distance to themeasurement object 90 is thus ascertained by a computing unit 73.

The WFD beam 76 has at least one sampling light pulse (WFD pulse) whichis modulated such that its portion returning from the target object isprovided for being evaluated according to the waveform digitizing method(WFD method)—with temporal sampling of the returning pulse—(that is tosay can be sampled and evaluated according to the WFD method), and atleast one phase comparison light signal train which is modulated suchthat its portion returning from the target object is provided for beingevaluated according to the phase comparison method.

Detection can then involve carrying out a signal sampling foridentifying and determining a temporal position of the returning portionof the sampling light pulse, and—in parallel therewith—evaluation of thedetected returning portion of the phase comparison light signal trainaccording to the phase comparison method.

Waveform digitizing (WFD) is based on the combination of two basicprinciples for signal detection that are customary in distancemeasurement. The first basic principle is based on measurement signaldetection on the basis of the threshold value method, and the secondbasic principle is based on signal sampling with downstream signalprocessing for identifying and determining the temporal position of thesignal. The propagation time and thus the distance between a measurementpulse emitted by the emission unit 71 and a target pulse (measurementpulse reflected from the target object 90 and detected by the detector72) follow for example from the temporal separation of the peak pointsof the two pulses, the pulses being sampled in a manner similar to thatin the case of phase measuring devices.

Advantageously, it is also possible to ascertain distances to moreremote targets than with the distance measuring devices shown in FIG. 5.

FIG. 6b shows a distance measurement to a retroreflector 61 (e.g. aspart of a measuring aid) by means of the ADM 13 of the coordinatemeasuring machine according to the invention. One exemplary manner offunctioning of such an ADM 13 has already been described with referenceto FIG. 5.

In the embodiment shown here, the measurement radiation 36 emerging fromthe beam source 131 of the ADM 13 is guided onto a polarizing beamsplitter 133 and from there via a second beam splitter 75 into thecommon beam path with the WFD beam and is thus directed through thelaser emission and reception optical unit 51 onto the retroreflector 61.The light returning on the same path is guided in the ADM 13 by thepolarizing beam splitter 133 onto an ADM detector 132. In principle, itis also possible to use other types of ADM, such as e.g. phase measuringdevices or distance measuring devices operating according to the Fizeauprinciple.

FIGS. 7 a, 7 b and 8 illustrate the beam receiving unit 80 in moredetail. FIG. 7a shows a first embodiment of the beam receiving unit 80.In this case, the reflected portion 81 of the WFD radiation that passedthrough the laser emission and reception optical unit into the beamdirecting unit is directed onto a parabolically shaped mirror 85, fromwhich the reflected radiation 81 is reflected in a manner concentratedat an angle of approximately 70° (in particular between approximately65° and approximately) 75° and is guided as concentrated light 82 via aplane mirror 86 onto a coupling-in unit 88 situated at the focal point,where the concentrated light 82 is coupled into the second opticalwaveguide 89, such that the light can be guided to the distancemeasuring device of the WFD module.

The second embodiment of the beam receiving unit 80 as shown in FIG. 7bcomprises a beam splitting apparatus 83, by means of which the reflectedradiation 81 is guided out of the beam path of the measurementradiation, such that the parabolic mirror 85 itself is not situated inthe beam path and can advantageously be totally non-transmissive to themeasurement beams.

As a result of the configurations of the beam receiving unit 80 asillustrated in FIGS. 7a -b, said beam receiving unit is positionable ina particularly space-saving manner in the beam directing unit.

FIG. 8 shows one exemplary embodiment of the paraboloidal mirror 85 in athree-dimensional view. The reflected radiation 81 is reflected from theparabolically shaped mirror surface in a concentrated manner in thedirection of the planar mirror.

FIG. 9 shows a second embodiment of an optical construction of a lasertracker according to the invention. In contrast to the constructionshown in FIGS. 6a and 6 b, the construction illustrated hereadditionally comprises an interferometer 12 as was described withreference to FIG. 5. The illustration shows simultaneous distancemeasurement to a retroreflector 61 by means of the ADM 13 and theinterferometer 12. The WFD module 70 is not used here, since the otherdistance measuring devices 12, 13 yield more accurate results in eachcase. Nevertheless, the WFD module 70 can also be used for measuring adistance to a retroreflector 61. What is advantageous here, inparticular, is the possibility of measuring, using the WFD radiation 76,distances over greater ranges than with the ADM 13 or the interferometer12.

FIG. 10 illustrates the second embodiment of the optical construction,wherein a distance measurement is carried out simultaneously with theWFD module 70 and the other distance measuring devices (ADM 13,interferometer 12), which can be used for calibrating the components ofthe WFD module 70 or the other distance measuring devices.

FIGS. 11 and 12 show two further embodiments of the optical constructionof a laser tracker according to the invention. These comprise, alongsidethe ADM 13, in each case an FMCW module 170 comprising a second beamsource 171 designed for emitting a frequency-modulated laser beam 176,thereby enabling a distance measurement by means of an FMCW method. Inparticular, the FMCW module can comprise a coherent laser radar, asdescribed e.g. in EP 1 869 397 B1.

The approach—used in this embodiment—for measuring the distance to adiffusely scattering surface of a measurement object 90 consists inemitting frequency-modulated electromagnetic radiation, such as e.g.light, onto the target to be measured, and subsequently in receiving oneor more echos from backscattering objects, ideally exclusively from thetarget to be measured. After reception, the possibly superimposed echosignal is superimposed with a mixing signal and the signal frequency tobe analyzed is thereby reduced, with the result that only a lower outlayis necessary in terms of apparatus. In this case, the mixing can becarried out either as a homodyne method with the transmitted signal oras a heterodyne method with a periodic, in particular harmonic, signalhaving a known period. Consequently, the methods differ in that mixingis carried out with the transmission signal itself or with a harmonicsignal with a dedicated frequency. The mixing serves to transform thereceived signal to lower frequencies. Afterward, the propagation timesand thus—given a known speed of propagation of the radiation used—thedistances to the targets to be measured are determined from theresulting signal.

The apparatuses used for implementing these methods usually use a signalgenerator as chirp generator, which impresses a signal on a modulatableradiation source. In the optical field, lasers are usually used asradiation sources. For emission and for reception, in the optical fieldit is possible to use transmission and reception optical units,downstream of which is disposed a detector or receiver with subsequentmixer, A/D converter and digital signal processor.

A linearly frequency-modulated chip is usually generated as signal s(t)by the signal generator:

s(t)=a+b·cos(2π·Φ(t)), Φ(t)=c+d·t+e·t ²  (1)

wherein the instantaneous frequency

${f(t)} = \frac{d\; {\Phi (t)}}{dt}$

is then a linear function of time:

f(t)=d+2e·t  (2)

which simplifies the determination of the propagation times.

In the case of n targets with relative amplitudes A_(k) and propagationtimes t_(k) (k=1, . . . , n), the noise-free echo signal e(t) can bewritten as follows:

$\begin{matrix}{{e(t)} = {\sum\limits_{k = 1}^{n}\; {A_{k}{s\left( {t - t_{k}} \right)}}}} & (3)\end{matrix}$

This echo signal e(t) is detected and mixed with the signal m(t):

$\begin{matrix}{{m(t)} = \left\{ \begin{matrix}{{{s\left( {t - t_{0}} \right)},}} & {{homodyne}\;} \\{{\cos \left( {2{\pi \left( {{f_{o}t} + \phi} \right)}} \right)},} & {heterodyne}\end{matrix} \right.} & (4)\end{matrix}$

Mixing with m(t) gives rise to the signal

$\begin{matrix}{{d(t)} = {\int\limits_{0}^{\infty}{{{h\left( {t - t^{\prime}} \right)} \cdot {e\left( t^{\prime} \right)} \cdot {m\left( t^{\prime} \right)}}{dt}^{\prime}}}} & (5)\end{matrix}$

where h denotes the impulse response of a suitable low-pass filter.

For an ideal low-pass filter, it is possible to explicitly implement thelow-pass filtering in (5) in accordance with the prior art to a verygood approximation; in the homodyne case, for example, there followsfrom the first equation (1) and equations (3) to (5) after omission ofthe high-frequency terms

$\begin{matrix}{{d(t)} = {d_{0} + {\frac{b^{2}}{2}{\sum\limits_{k = 1}^{n}\; {A_{k}\mspace{14mu} {\cos \left( {2{\pi \left\lbrack {{\Phi \left( {t - t_{k}} \right)} - {\Phi \left( {t - t_{0}} \right)}} \right\rbrack}} \right)}}}}}} & \left( 5^{\prime} \right)\end{matrix}$

with the signal offset

$d_{0} = {a^{2}{\sum\limits_{k = 1}^{n}\; {A_{k}.}}}$

The mixed signal d(t) is digitized on the finite measurement interval−T/2≤t≤T/2 and stored.

From the frequency information and, if appropriate, the phaseinformation of this signal, the propagation times t_(k) are determined,wherein n can normally be assumed to be small and possibly also to beknown. One of the echoes, e.g. the n-th, can also originate from a fixedand known reference target and the target distances of the remainingtargets are calculated from the propagation time differences t_(k)−t_(n)and the known distance of the reference target. In the homodyne case,the mixing signal m(t)=s(t−t₀) itself can serve as a reference; t₀ thencorresponds to the reference distance.

In the case of a linear chirp in accordance with equation (1), the k-thecho contributes the instantaneous frequency

$\begin{matrix}{{f_{k}(t)} = \left\{ \begin{matrix}{{{2{e\left( {t_{0} - t_{k}} \right)}},}\mspace{79mu}} & {{homodyne}\;} \\{{d + {2{e\left( {t - t_{k}} \right)}} - f_{0}},} & {heterodyne}\end{matrix} \right.} & (6)\end{matrix}$

to the signal d(t). Consequently, in this case, the propagation timest_(k), can be determined directly, in principle, from a frequencyanalysis—which is temporally resolved in the heterodyne case—of thesignal d(t), although the resolution is still coarse. More accurateresults can be obtained by taking account of the phase information.

A frequency-modulated method with continuous emission (FMCW method) fordistance measurement with electromagnetic radiation in the radar rangeis known from DE 196 10 970 A1. In that case, a time-linearlyfrequency-modulated (wobbled) signal is emitted and is analyzed afterreflection at a target and subsequent reception. In that method, anintermediate frequency signal is generated from transmission andreception signals in a mixer and is subjected to a fast Fouriertransformation.

FIG. 11 shows measurement of a distance to a measurement object 90having a diffusely scattering surface by means of the FMCW method.

An FMCW module 170 is installed together with the ADM 13 in the beamdirecting unit of the laser tracker and uses the same laser emissionoptical unit 51 a as the ADM 13. The FMCW module 170 comprises a beamsource 171 with drive means for generating a frequency-modulated laserbeam 176 having a continuously variable frequency. Thefrequency-modulated laser beam 176 is firstly guided onto a first beamsplitter 174, whereby a reference beam 177 is split off and is guidedonto a mixer element 178.

The other part of the frequency-modulated laser beam 176 is coupled intothe common beam path with the measurement radiation of the ADM 13 bymeans of a second beam splitter 175 and is directed through the laseremission optical unit 51 a onto the remote measurement object 90 to bemeasured. Radiation 181 of the frequency-modulated laser beam 176 thatis reflected from the diffusely scattering surface of the measurementobject 90 passes partly through the laser reception optical unit 51 b(embodied here separately from the laser emission optical unit 51 apurely by way of example) again to the beam directing unit. In thelatter, the reflected radiation 181 is guided onto the mixer element178, in which the reflected radiation 181 is subjected to homodyne orheterodyne mixing with the reference beam 177. Optionally, the reflectedradiation 181 may have been amplified beforehand by means of an RFpreamplifier.

The resulting mixed signal 182 is then passed to the detector 172 of theFMCW module 170; in particular, the mixed signal can in this case alsobe passed via a low-pass filter and a baseband amplifier to the detector172, which can be designed as an analog/digital converter, inparticular. A computing unit 173, in a known manner, can then ascertaina frequency difference, i.e. in particular a separation between thefrequencies of the reference beam 177 and the reflected radiation 181,and calculate therefrom the distance to the measurement object 90.

FIG. 12 shows an alternative embodiment of the FMCW module 170. Thelatter comprises, instead of the mixer element, an optical referencesystem for the reference beam 177 having two optical fibers 184 a, 184b. Such an optical reference system is described in EP 1 869 397 B1, forexample. In this case, the first optical fiber 184 a has a differentlength than the second optical fiber 184 b. The reference beam 177 issplit in a first optical fiber coupler 183, guided as respective partsthrough the first and second optical fibers 184 a, 184 b and, havingbeen combined again by means of a second optical fiber coupler 185, isguided onto a reference beam detector 186. On account of the knowndifferent lengths of the two optical fibers 184 a, 184 b, the frequencyof the reference beam 177 can be accurately detected by the referencebeam detector 186 at any point in time. The reflected portion 181 of thefrequency-modulated laser beam 176 is guided via the coupling-in unit188 and the optical waveguide 189 onto the detector 172 (e.g.analog/digital converter). On the basis of the frequencies of thereflected radiation 181 and of the reference beam 177, or on the basisof the temporal offset thereof, the distance to the target is calculatedin the computing unit 183 in a known manner.

In the embodiments of the FMCW module 170 as illustrated in FIGS. 11 and12, the reception of the reflected radiation 181 in the beam directingunit can optionally also be carried out via a beam receiving unit asillustrated e.g. in FIGS. 7a and 7 b. Likewise, the laser emissionoptical unit 51 a and the laser reception optical unit 51 b can also beembodied as a common laser emission and reception optical unit.

In a further embodiment, illustrated in FIG. 13, at least the first orsecond distance measuring device comprises means which enablemeasurement of distances that is based on a frequency comb or issupported by a frequency comb. For this purpose the correspondingdistance measuring device comprises a laser source designed for emittinga pulsed, highly precisely timed femtosecond laser having a carriersignal. As a result, a so-called frequency comb of thin sharp lines canbe generated in the frequency range, which frequency comb can be usedfor a precise optical frequency measurement. Various approaches for thefrequency-comb-based or frequency-comb-supported measurement ofdistances are described for example in the paper “Frequency-Comb BasedApproaches to Precision Ranging Laser Radar” (N. R. Newbury, T.-A. Liu,I. Coddington, F. Giorgetta, E. Baumann, W. C. Swann; National Instituteof Standards and Technology). FIG. 13 illustrates purely by way ofexample an interferometer unit 300 with two frequency comb laser signals360, 370 (“dual-comb interferometer”) for carrying out a combinedtime-of-flight and interferometric distance measurement. In this case, afirst beam source 310 emits a first frequency comb signal 360. Thelatter is guided via a first beam splitter 330 and via a reference beamsplitter 340 onto a retroreflector as measurement target 61. Thecombined signal 380 reflected from the measurement target 61 andreference 360 is guided onto a frequency comb analysis unit 350, wherethe relative arrival time of the pulses reflected from measurementtarget 61 and reference 360 can be used for a time-of-flight distancemeasurement. A reference frequency comb signal 370 which is generated bya second beam source 320 and which has a repetition rate deviatingslightly from the first frequency comb signal 360 can be read in aheterodyne manner in the frequency comb analysis unit 350 together withthe reflected combined signal 380 for an interferometric distancemeasurement. In a laser tracker according to the invention, both thefirst and the second distance measuring units can be designed forfrequency-comb-based or frequency-comb-supported measurement of adistance to a target. Besides the embodiment illustrated purely by wayof example in FIG. 13, in particular a WFD module or FMCW moduleaccording to the invention of a laser tracker can also be designed in afrequency-comb-supported manner.

It goes without saying that these illustrated figures merelyschematically illustrate possible exemplary embodiments. According tothe invention, the various approaches can likewise be combined with oneanother and also with systems and methods for measuring surfaces and/orobjects and with measuring instruments from the prior art.

What is claimed is:
 1. A laser tracker comprising: a base, a support,which is fixed on the base rotatably about a first rotation axis, a beamdirecting unit, which is fixed to the support rotatably about a secondrotation axis, which is substantially orthogonal to the first rotationaxis, means for detecting a rotation angle of the support relative tothe base, and means for detecting a rotation angle of the beam directingunit relative to the support, wherein the beam directing unit comprises:a laser emission and reception optical unit for emitting measurementradiation and for receiving measurement radiation reflections, a firstoptical distance measuring unit having at least one first distancemeasuring device for measuring the distance to the retroreflector bymeans of a first measurement radiation, and a second optical distancemeasuring unit comprising a second beam source for emitting a secondmeasurement radiation, a detector and a control and processing unit formeasuring a distance to a diffusely scattering surface of a targetobject by means of the second measurement radiation, wherein: the secondbeam source comprises drive means and is designed to generate afrequency-modulated laser beam having a continuously varying frequencyas second measurement radiation, a part of the second measurementradiation can be split off as reference beam, a portion of the secondmeasurement radiation that is reflected from the surface of the targetobject is forwardable to the detector, and the second optical distancemeasuring unit is designed to ascertain a distance to the surface of thetarget object on the basis of a frequency evaluation of the referencebeam and the measurement radiation reflection, a distance to the surfaceof the target object.
 2. The laser tracker according to claim 1,wherein: the second measurement radiation can be split off as referencebeam by means of a first beam splitter provided in the beam path of thesecond measurement radiation in the beam directing unit.
 3. The lasertracker according to claim 1, wherein: the second optical distancemeasuring unit is designed to ascertain a distance to the surface of thetarget object on the basis of the principle of the evaluation of a phasedifference between the frequency modulation of the reference signal andthe frequency modulation of the reflected measurement radiation signal4. The laser tracker according to claim 1, wherein the second opticaldistance measuring unit comprises: an optical reference system having afirst optical fiber, a second optical fiber and a reference beamdetector, wherein the first and second optical fibers in each case havea known, mutually different length, wherein the two lengths differ fromone another in a ratio of at least 2:1, and the reference beam isforwardable to the optical reference system; or a mixer, wherein thereference beam and the reflected measurement radiation reflection areforwardable to the mixer, and the mixer is designed for carrying out ahomodyne or heterodyne mixing method, wherein the second opticaldistance measuring unit comprises a low-pass filter disposed downstreamof the mixer.
 5. A laser tracker comprising: a base, a support, which isfixed on the base rotatably about a first rotation axis, a beamdirecting unit, which is fixed to the support rotatably about a secondrotation axis, which is substantially orthogonal to the first rotationaxis, means for detecting a rotation angle of the support relative tothe base, and means for detecting a rotation angle of the beam directingunit relative to the support, wherein the beam directing unit comprises:a laser emission and reception optical unit for emitting measurementradiation and for receiving measurement radiation reflections, a firstoptical distance measuring unit having at least one first distancemeasuring device for measuring the distance to the retroreflector bymeans of a first measurement radiation, and a second optical distancemeasuring unit comprising a second beam source for emitting a secondmeasurement radiation, a detector and a control and processing unit formeasuring a distance to a diffusely scattering surface of a targetobject by means of the second measurement radiation, wherein: the firstoptical distance measuring unit and/or the second optical distancemeasuring unit are/is designed for a frequency-comb-based orfrequency-comb-supported distance measurement.
 6. The laser trackeraccording to claim 5, wherein: a scanning functionality, in the contextof which the laser tracker is designed to carry out, by means of thesecond measurement radiation, a multiplicity of distance measurementswith respect to a multiplicity of measurement points on the surface ofthe measurement object, the control and processing unit is designed insuch a way that for the multiplicity of the distance measurements, therotation angles respectively detected are linked with the measureddistance, such that a point position is in each case defined by thelinking, and a point cloud comprising a number of the point positions isgeneratable.
 7. The laser tracker according to claim 5, furthercomprising: a calibration functionality for calibrating the firstoptical distance measuring unit and/or the second optical distancemeasuring unit, wherein: the control and processing unit is designed insuch a way that the first measurement radiation and the secondmeasurement radiation are emittable in a temporal relationship onto aretroreflector, and the first optical distance measuring unit and/or thesecond optical distance measuring unit are/is calibratable on the basisof a first distance to the retroreflector as measured by the firstoptical distance measuring unit and a second distance to theretroreflector as measured by the second optical distance measuringunit.
 8. The laser tracker according to claim 5, wherein: the beamdirecting unit comprises a position-sensitive detector for detecting thefirst measurement radiation or second measurement radiation reflected bya target, such that depending on a position of the reflected measurementradiation on the detector the alignment of the measurement radiation isreadjustable for a continuous target tracking, wherein the beamdirecting unit additionally comprises a localization camera for thecoarse localization of the measuring aid and/or an overview camera forproviding images for a user; the first optical distance measuring unitcomprises an interferometer and/or an absolute distance measuringdevice; the beam directing unit comprises deflection means forgenerating a common beam path of the first measurement radiation and thesecond measurement radiation in the beam directing unit, such that thefirst measurement radiation and the second measurement radiation areemittable through the same laser emission and reception optical unit;the laser emission and reception optical unit is designed as a laseremission optical unit and a laser reception optical unit separate fromthe latter; and/or the beam directing unit comprises two laser emissionand reception optical units, the first measurement radiation isemittable through the first laser emission and reception optical unit,and the second measurement radiation is emittable through the secondlaser emission and reception optical unit.
 9. A method for using an FMCWmodule in a laser tracker for providing additional measurementfunctionality, wherein the laser tracker comprises a beam directing unithaving: a laser emission and reception optical unit for emittingmeasurement radiation and for receiving measurement radiationreflections, and an optical distance measuring apparatus having at leastone first distance measuring device for measuring the distance to ameasuring aid by means of a first measurement radiation, wherein themethod comprises: generating a frequency-modulated laser beam having acontinuously variable frequency as second measurement radiation by meansof a beam source of the FMCW module, emitting the second measurementradiation onto a target to be measured, wherein one part of the secondmeasurement radiation is split off as reference beam, and another partof the second measurement radiation is emitted through the laseremission and reception optical unit, a portion of the second measurementradiation that is reflected from the target and received through thelaser emission and reception optical unit is forwarded to a detector ofthe FMCW module, and a distance to the target is ascertained on thebasis of a frequency evaluation of the reference beam and the reflectedportion of the second measurement radiation.
 10. The method according toclaim 9, wherein: the distance to the target is ascertained on the basisof the principle of the evaluation of a phase difference between thefrequency modulation of the reference signal and the frequencymodulation of the reflected measurement radiation signal.
 11. The methodaccording to claim 9, wherein: the target to be measured is a diffuselyscattering surface of a measurement object, wherein: the secondmeasurement radiation is emitted onto the diffusely scattering surface,a portion of the second measurement radiation is reflected from thesurface, and the distance to the surface is ascertained, wherein: amultiplicity of distance measurements with respect to a multiplicity ofmeasurement points on the surface are carried out by means of the secondmeasurement radiation, for the multiplicity of the distance measurementsa present alignment of the beam directing unit is linked with therespectively measured distance, such that a point position is in eachcase defined by the linking, and a point cloud comprising a number ofthe point positions is generated.
 12. The method according to claim 9,wherein: the target to be measured is a retroreflector the secondmeasurement radiation is emitted onto the retroreflector, the secondmeasurement radiation is reflected as measurement radiation reflectionfrom the retroreflector, and the distance to the retroreflector isascertained, wherein the first measurement radiation and the secondmeasurement radiation are emitted onto the retroreflector, wherein adistance measured by means of the first measurement radiation and adistance measured by means of the second measurement radiation are usedfor calibrating the first optical distance measuring device and/or theWFD module or the FMCW module, wherein the first measurement radiationand the second measurement radiation are emitted simultaneously.
 13. Anon-transitory computer program product having program code, stored on amachine-readable carrier, for controlling or carrying out the method ofclaim
 9. 14. A laser tracker comprising: a base, a beam directing unitfor directing or deflecting measurement radiation, wherein it isarranged rotatably about a first and about a second rotation axisrelative to the base, means for detecting a first and second rotationangle of the beam directing unit, and an optical distance measuring unithaving at least one first distance measuring device for measuring thedistance to the retroreflector by means of a first measurementradiation, and wherein: the distance measuring unit is designed forfrequency-comb-based or frequency-comb-supported distance measurement.